The present invention relates to a method for applying a magnetic liquid including at least a magnetic substance and a binder to a flexible carrier (which is hereinafter often referred to as web) such as a plastic film, paper or a metal leaf.
Conventional application methods in which a liquid is applied to a web are generally practiced with an application device of the extrusion type, an application device of the curtain flow type, an application device of the doctor blade type, an application device of the slide coating type, etc. The application method practiced with the application device of the extrusion type is capable of applying a liquid to the web to form a uniform thin film thereon, and is used in various fields, as described in the Japan Patent Applications (OPI) Nos. 84771/82, 104666/83 and 238179/85 (the term "OPI" as used herein means an "unexamined published application"). However, the conditions for good application in the method which is practiced with the application device of the extrusion type are limited within narrow ranges.
In recent years, the density of recording in a magnetic recording medium and the number of the layers thereof have been increased. For that reason, it has been required that the thickness of a magnetic layer on a nonmagnetic carrier be decreased in manufacturing the medium. The speed of application of a liquid to the carrier has been desired to be higher to enhance the productivity for the medium. Magnetic substances have been improved so that a magnetic oxide powder of high S.sub.BET value and using barium ferrite have come into use. As a result, the viscosity of the applied liquid has increased. This has resulted in a problem in that it is difficult to obtain conditions for good application of the liquid, and the surface of the film of the liquid applied on the web is deteriorated due to the high cohering property of the liquid, making it impossible to render the quality of the film stable and good.
To solve this problem, an application device, the flow property of an applied liquid in the slot of which is controlled to improve the properties of the magnetic recording medium, particularly the electromagnetic conversion property thereof, has been proposed, as disclosed in the Japan Patent Application (OPI) No. 189369/89. The flow property of the applied liquid is set in accordance with a flow index based on the mean speed of the flow of the liquid in the slot and the mean viscosity thereof in the slot, to thereby establish the design factors of the application device.
However, with the use of the application device disclosed in Japanese Patent Application (OPI) No. 189369/89, a good film cannot necessarily be formed from the applied liquid. Particularly, the higher the S.sub.BET value of the magnetic substance of the liquid is set (45 m.sup.2 /g or more) to increase the viscosity thereof, the harder it is to obtain a desired electromagnetic conversion property. This is a significant problem.
The present inventors conducted intensive studies on application factors which determine the properties of the film of the applied liquid, particularly, the electromagnetic conversion property thereof. As a result, they found that although the flow property of the applied liquid in the slot is important, what is decisively important is the flow property of the liquid on the surface of a doctor edge portion. In other words, even if the flow property of the applied liquid in the slot is predetermined, the flow property changes on the surface of the doctor edge portion due to the re-cohering property of the liquid or the like, as a result of which minute streaks occur in the surface of the magnetic layer of the magnetic recording medium. The occurrence of such streaking degrades the electromagnetic conversion property of the layer. Therefore, the flow property of the applied liquid on the surface of the doctor edge portion is decisively important.
The present invention further relates to an applicator device, and more particularly to a device for coating a magnetic liquid, which includes at least a magnetic substance and a binder, onto a flexible carrier, or web, made of a plastic film, paper, metal leaf, or the like.
Conventional methods for coating a liquid onto a web generally are practiced with applicators of the extrusion type, curtain flow type, doctor blade type, slide coating type, and so forth. The method which is practiced with the extrusion-type applicator is capable of applying the liquid to the web so as to provide a uniform thin layer of the liquid thereon. Accordingly, such an applicator has been used in various fields, as described in Japanese Patent Applications (OPI) Nos. 104666/83 and 238179/85, Japanese Patent Application No. 84711/89, among others.
Since the doctor edge portion of an application device disclosed in Japanese Patent OPI No. 104666/83 has two flat surfaces meeting each other and defining an obtuse angle therebetween, the doctor edge portion can be processed with high accuracy, and applied liquid can be pressed appropriately on a web. Also, the device copes well with fluctuations in the tension of the web and the like, air is prevented effectively from being entrained into the liquid at the time of rapid application thereof, and the nonuniformity of thickness of the film of the applied liquid on the web is suppressed. However, if the applicator operates at a relatively high speed, such as from about 200 m/min to about 300 m/min, a problem can arise in that foreign matter in the liquid is likely to be trapped at the top of the doctor edge portion, causing streaking in the film of the applied liquid on the web.
To solve this problem, an application device in which the positional relationship between the surface of a doctor edge portion and that of a back edge portion is set in a prescribed range and the curvature of the surface of the doctor edge portion is also set in a prescribed range has been proposed, as disclosed in the Japanese Patent OPI No. 238179/85. The surface of the doctor edge portion is curved so that the area of pressing of a web by the surface of the portion can be widened somewhat to prevent a streak from being caused due to a narrow area of pressing of the web. While this technique solves the aforementioned problem, an additional problem arises, for the following reason. It has been required recently that the speed of application of a liquid to a web be as high as 300 m/min or more, and that the thickness of the film of the applied liquid on the web be as small as 10 cc/m.sup.2 or less. As a result, entrainment of air into the film has been noticed again.
Under such circumstances, an application device has been proposed, as disclosed in Japanese Patent Application No. 84711/89. In that device, the radius of the curvature of the surface of a doctor edge portion is set in a prescribed range of small values such as 2 mm or less, so that the pressure of the surface on a liquid and a web is increased to prevent air from being involved into the film of the applied liquid on the web. However, a new problem arises in that, since the radius of the curvature of the surface of the doctor edge portion is made small, the length of the surface along the direction of the movement of the web also must be small. This means that, if the length of the surface of the doctor edge portion along the direction of the movement of the web is to be increased as the radius of the curvature of the surface is 5 mm or less, for example, then the angle between the inner surface of the portions of the web, which are bent from each other around the top of the doctor edge portion, needs to be decreased in order to augment the length. In that case, the load on the web needs to be made heavier than originally necessary due to the decrease in the angle, and the angle between the vertical surface of a back edge portion and the top thereof needs to be acute. As a result, it is likely that the web will be scraped or stretch--at best causing difficulty or lowering the quality of application and of the resulting product, and at worst causing the web to break.
Although the thickness of the film of the applied liquid on the web can be made uniform with these extrusion-type applicators, only a narrow range of conditions for good application is possible, as described above. Particularly in recent years, when the density of recording in a magnetic recording medium and the number of the layers thereof have increased so that the thickness of the magnetic layer on a nonmagnetic web needs to be decreased during manufacture of the medium, it is desirable, more so now than previously, that the speed of the application of a liquid to the web be heightened to enhance productivity of the medium.
Further, since magnetic substances have been improved to use a magnetic oxide powder of high S.sub.BET value and a barium ferrite to increase the density of recording in a magnetic recording medium, the viscosity of an applied liquid including such a substance is increased, causing a problem in that the state of the surface of the film of the applied liquid on a web cannot be improved with an applicator in which the length of the surface of the doctor edge portion along the direction of the movement of the web cannot be made sufficiently large. In other words, the problem is that it has been found through a microscope that the state of the surface has deteriorated due to the high cohering property of the liquid, making it impossible to render the film of the applied liquid on the web desirably stable.
In particular, a problem arises in that, the higher the S.sub.BET value of a magnetic substance included in an applied liquid (45 m.sup.2 /g or more) so as to raise the viscosity of the liquid, the harder it is to achieve a desired electromagnetic converting property.